Re: [Qemu-devel] [PATCH] mmap-alloc: use same backend for all mappings

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> On Mon, Nov 30, 2015 at 02:46:31PM +0100, Greg Kurz wrote:
>> On Mon, 30 Nov 2015 15:06:33 +0200
>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> 


....
>> 
>> On ppc64, the address space is divided in 256MB-sized segments where all pages
>> have the same size. This is a hw limitation IIUC. I don't know if it can be
>> fixed and I'll let Ben comment on it.
>
> But it's anonymous memory with PROT_NONE.  There should be no pages there:
> just a chunk of virtual memory reserved.
>

ppc64 use page size (called as base page size) to find the hash slot in
which we find the virtual address to real address translation. All the
pages in a segment should have same base page size. Hugetlb pages have a
base page size of 16M whereas a regular linux page have 64K. mmap will
fail to map a hugetlb mapping in a segment that already have regular
pages mapped.

-aneesh